Carbon nanotubes (CNT) in general exhibit exceptional electronic and mechanical properties. Therefore, carbon nanotubes are expected to find a big diversity of industrial applications. One of these applications could be the use as both passive and active components in nano-electronics. A lack of understanding of CNT growth mechanisms, however, presents a significant challenge to the realization of such applications. One of the key issues in these growth mechanisms is the formation of catalyst particles (also referred to in this application as nanoparticles) with uniform and controllable diameter to be used in e.g. catalyst mediated chemical vapor deposition processes for CNT growth. Control of the diameter size and uniformity of catalyst nanoparticles is very critical to obtain uniform CNT with controllable diameter.
On the other hand the use of CNT in e.g. electronic applications implies, in some cases, the use of pristine Si as substrate. That is, depositing a catalyst and growing the CNT, directly on Si. However, the associated temperature of typical CNT growth processes reported so far in the state of the art produce a reaction between the metallic catalyst, which may typically be Fe, Co or Ni, and the Si substrate. Thus, the as-prepared catalyst nanoparticles change from pure metal to metal silicide. Several such CNT growth studies have been reported and the catalytic activity of these metal silicides is still under debate. It is not yet clear why these nanoparticles are frequently reported as being inactive, while they have been demonstrated to be active as pure metal. This raises the question of whether or not metal silicide blocks CNT growth.
One of the first reports (Appl Phys. Lett. 77 (2000) 2767) involved sputtering a thin layer of metal catalyst, in the example given a Co layer, onto a Si(100) substrate. Cobalt silicide formation was observed at the Co—Si interface at 825° C. indicating that the cobalt reacts with the silicon during the process.
In “Influence of iron-silicon interaction on the growth of carbon nanotubes produced by chemical vapor deposition”, Appl. Phys. Lett. 80(13), (2002), page 2383, T. de los Arcos et al. described that undesired interaction of the metallic catalyst with the silicon substrate, hereby forming metal silicide, could deteriorate the catalytic efficiency of the particles formed for CNT growth. It was shown that, after heating up to 850° C., a silicon substrate comprising a thin iron layer was turned into a silicon substrate having iron silicide particles on top. Subsequent CNT growth using the silicide particles as a catalyst lead to a low density of CNTs on the substrate compared to CNTs grown on a substrate having a barrier layer in between the silicon and the metallic layer to form catalyst, in the example given iron, particles. Furthermore, CNT growth using the iron silicide particles as a catalyst was much slower than when the iron particles formed on the barrier layer were used as a catalyst. Therefore, it was concluded that formation of silicides at the metal/silicon substrate interface should be avoided in order not to decrease the catalyst activity of the formed catalyst particles and thus that metal-silicide particles are not suitable as a catalyst in CNT growth.
In the case of Ni (Appl. Phys. Lett. 79 (2001) 1534), it is described to use a diffusion barrier between the underlying Si substrate and the metal for catalysis of CNT growth. This sample configuration was reported to maintain “active” Ni particles for CNT nucleation and growth by explicitly preventing the formation of Ni-Silicides above 300° C. However, no direct evidence of “catalyst inactivity” in the absence of the diffusion barrier was presented or cited. The same research group later reported the occurrence of silicidation when thin films of Ni and Co were deposited onto three different Si substrates: untreated Si with a thin native oxide, pristine Si and Si with 50 nm of SiO2 (J. Appl. Phys. 90 (2001) 5308). Due to silicidation, no islands were found after annealing at 750° C. in the case of samples where Ni was deposited on untreated Si or Si with native oxide. In contrast, Ni nanoparticles were identified following similar annealing of the Ni deposited on SiO2. Further, on the latter it was possible to grow CNT. Thus, it was concluded that a barrier layer such as SiO2 is required to prevent silicide formation when Ni is used as catalyst. This work was followed by several others claiming the need for a barrier between Ni and a Si substrate.
Overall, similar results but varying conclusions have been shown for catalyst systems combining metals and Si substrates that potentially form metal silicide. On the one hand, CNT growth is catalyzed by these metal compound nanoparticles. On the other hand, the absence of growth has been reported in some systems under almost identical sample preparation and growth conditions (see above).